Activation of platelets by vessel wall proteins at sites of plaque rupture is a key event in the pathogenesis of stroke and myocardial infarction. Collagen is exposed to platelets by the rupture of atherosclerotic lesions and is a powerful activator of platelets. Collagen activation of platelets is mediated by two receptors, the integrin alpha2beta 1 and the non-integrin receptor GPVI, but precisely how platelets are activated by these receptors is not understood. We have developed a heterologous expression system in which collagen signaling is conferred by expression of both alpha2beta1 and GPVI but not by either receptor alone. These results are consistent with genetic and pharmacologic studies demonstrating that loss of alpha2beta1 or GPVI results in loss of platelet activation by collagen. We hypothesize that collagen signaling in platelets is the result of sequential interactions between collagen, alpha2beta 1 and GPVI during which one or both receptors transmit signals and that the activation of platelets by collagen is a rate-limiting step in arterial thrombosis. Our specific aims are: 1. To determine whether alpha2beta1 and GPVI interact directly during collagen signaling. 2. To delineate the binding sites for collagen and the GPVI co-receptor Fc Ralpha on GPVI. 3. To determine whether alpha2beta1 activates P13 kinase, small G-protein and MAPK signaling pathways in response to collagen. 4. To determine whether elevated levels of platelet alpha2beta1 and/or GPVI-Fc Rgamma augment collagen responses and accelerate thrombosis in vivo using a transgenic mouse model system. We will address these questions with biochemical and signaling studies of wild-type and mutant receptors expressed in vitro using a heterologous expression system and in vivo using transgenic and gene-targeted mouse models. The role of collagen receptors in regulating arterial thrombosis in vivo will be addressed with arterial thrombosis assays performed on transgenic mice overexpressing collagen receptors. These studies will help define the pathogenesis of stroke and myocardial infarction at the molecular level and will lay the foundation for novel therapies to treat these diseases.